The earliest mention of Ninian of Whithorn is in a short passage of The Ecclesiastical History of the English People by the Northumbrian monk Bede in ca. 731. The 9th-century poem Miracula Nyniae Episcopi records some of the miracles attributed to him. A Life of Saint Ninian (Vita Sancti Niniani) was written around 1160 by Ailred of Rievaulx, and in 1639 James Ussher discusses Ninian in his Brittanicarum Ecclesiarum Antiquitates. These are the sources of information about Ninian of Whithorn, and all provide seemingly innocuous personal details about his life. However, there is no unchallenged historical evidence to support any of their stories, and all sources had political and religious agendas that were served by their accounts of Saint Ninian (discussed below).
Tradition holds that Ninian was a Briton who had studied in Rome, that he established an episcopal see at the Candida Casa in Whithorn, that he named the see for Saint Martin of Tours, that he converted the southern Picts to Christianity, and that he is buried at Whithorn. Variations of the story add that he had actually met Saint Martin, that his father was a Christian king, and that he was buried in a stone sarcophagus near the altar of his church. Further variations assert that he left for Ireland, and died there in 432. Dates for his birth are derived from the traditional mention of Saint Martin, who died in 397.